KEY POINTS
- Investigators generated 92 independently optimized VMAT plans for 23 patients with liver metastases, comparing four Monaco XVMC configurations: 1%/2 mm, 1%/3 mm, 2%/2 mm and 2%/3 mm statistical-uncertainty/grid combinations.
- Patients were planned to 60 Gy in eight fractions with a 115–120% GTV simultaneous integrated boost (69–72 Gy) using two partial 6-MV VMAT arcs.
- Increasing the dose grid from 2 to 3 mm reduced final Monte Carlo calculation time by roughly threefold: from 2,091 to 676 seconds at 1% uncertainty and from 1,490 to 439 seconds at 2% (p<0.001).
- The faster calculation came with changes in apparent target dosimetry. At 1% uncertainty, reported GTV D98 increased from 70.44 to 71.47 Gy, but this reflected coarser spatial sampling and volume averaging rather than demonstrated improvement in true coverage.
- The 3-mm grid also produced less favorable conformity, homogeneity and R50, while higher statistical uncertainty increased small-volume GTV maximum dose and D2cm. Global gradient index did not significantly change.
- Most OAR differences were numerically small compared with between-patient variation, suggesting the principal impact was on high-gradient target and dose-falloff metrics rather than broad normal-tissue exposure.
- Patient-specific QA did not distinguish the four calculation settings: gamma passing rates did not differ significantly at 3%/3 mm or 2%/2 mm. Yet only 12/20 measurements at 3%/3 mm and 10/20 at 2%/2 mm exceeded the prespecified 95% threshold, and QA was performed in only five patients.
CLINICAL TAKEAWAY
For liver SBRT, Monte Carlo calculation settings are not just a speed preference: changing grid size or statistical uncertainty can alter the DVH metrics used to judge plan quality. The study supports retaining fine-resolution calculation for high-gradient stereotactic planning and reinforces that gamma QA alone cannot establish dosimetric equivalence between calculation settings.